Probably in the region of €6,000 - €6,500.00. This would include new single coil factory insulated cylinder, immersion heater, immersion timeclock, 3 zone sytem each with time & temperature control, Vokera HE gas boiler, pipework, radiators, TRV's, etc. It would also include using all good quality materials.
Allow an extra €275 for Bord Gais install and connect gas meter. This includes upto 15m from road to meter. If it is further, they have a per meter rate for every extra meter.
This is just a guide price obviously but it should not be anymore than this and it could be less. You will also qualify for the SEAI grant of €610.00 for the works (€400 for heating controls, €160 for new HE boiler & €50 for BER following works). Obviously you will have to use an SEAI registered contractor to avail of the grant. Don't let installers tell you it is cheaper to avoid the grant. You MUST install to this standard to meet building regulations, so if you have to anyhow, why not avail of the grant. You will also have peace of mind that the installer will be audited independently and any errors found will have to be corrected by them at their expense, so it's of some assurance for you. It is also better to have full time & temperature control as it is much cheaper to operate the system.
This price indication is also for heating system only. Domestic side will depend largely on the specification you want. For example, you can pay €15 for a pair of taps but you can also pay €500 for a pair. It is better if you purchase your own sanitary ware, sinks, showers, etc. and get a price for install only.